Solar Panel Shading
The silent output killer

The chain effect — how one panel drags down the rest

In a standard string inverter setup, solar panels are wired in series — like old Christmas lights. The current flowing through the string is limited to the weakest panel. If one panel produces less power due to shade, dirt, or a fault, the entire string drops to match.

The numbers are dramatic: a 30% shaded cell on just one panel in an 8-panel string can cut the whole string's output by 30–70%, depending on the inverter's MPPT algorithm and the severity of the shading.

This is why a seemingly minor obstruction — a TV dish, a chimney, a single tree branch — can have an outsized effect on your system's production. It's not about the area of shadow; it's about which cells are affected and how many panels share that string.

Types of shading

• Hard shading: trees, chimneys, TV dishes, water tanks, parapet walls, neighbouring buildings. These cast defined shadows that move predictably through the day. Hard shading is the most damaging type and the most avoidable during design.
• Soft shading: clouds, haze, air pollution, light dust. Unavoidable and manageable. Correctly sized systems account for soft shading in their production estimates.
• Self-shading: panels shading each other. A real risk on flat roofs where tilt frames are installed too close together. In winter, when the sun is lower (especially in Cape Town at 34°S), front rows can cast shadows on rear rows for hours. Minimum spacing between rows depends on latitude and tilt angle (see optimal tilt settings in our panel angle and orientation guide).
• Seasonal shading: a tree that clears your panels in December (summer, high sun) may cast a long shadow across them in June (winter, low sun). Always assess shading at the worst time of year: June–July for most of SA.

How to assess shading before installation

You don't need expensive equipment. Here's a practical approach:

1. Walk your roof at solar noon in winter (June, between 12:00 and 13:00). Note anything that casts a shadow on where panels would go.
2. Use a free app: SunSurveyor or SunSeeker shows the sun's exact path at your GPS location for any date. Point your phone camera at the roof area and see where the sun will be in every month.
3. Note any obstructions within a 45° cone of the intended panel field — trees, aerials, chimneys, neighbouring walls.
4. Ask your installer to model shading in PVsyst, PV*SOL, or similar design software. Any professional installer should do this for free as part of quoting. If they don't, that's a red flag.

Technology solutions

String inverters (standard)

The cheapest and most common inverter type in SA. All panels on a string must be on the same roof plane at the same angle. If any panel is shaded, the whole string suffers.

Best for: Completely unshaded roofs with a single north-facing plane. This covers about 60% of SA suburban homes.

SA price range: R8 000–R18 000 for 5–8 kW (e.g., Deye, SunSynk, Growatt hybrid models — these are technically hybrid inverters but function as string inverters for the PV input).

Power optimisers (SolarEdge, Tigo)

Small devices attached to each panel. Each panel operates at its own maximum power point independently, so one shaded panel doesn't drag the string down. The output is fed to a central inverter.

Cost premium: R800–R1 500 per panel on top of the inverter cost. For a 10-panel system, that's R8 000–R15 000 additional.

Worthwhile if: You have partial shading on an otherwise good roof — a chimney shadow, a tree that can't be removed, or panels on two different roof angles.

Microinverters (Enphase)

Each panel has its own small inverter. Maximum flexibility — every panel is completely independent. Best shading tolerance and best performance on complex roofs with multiple faces and orientations.

Cost premium: R2 000–R3 500 per panel versus a string inverter equivalent system. For a 10-panel system, that's R20 000–R35 000 additional — significant.

Best for: Complex roofs, heavy partial shading, or systems where future expansion with different orientations is planned.

Dust, soiling, and self-cleaning

South African dust — especially in the Northern Cape, Gauteng's Highveld during winter dry season, and Free State farming areas — deposits on panels and reduces output by 3–8% per month without rain.

The fix is tilt angle: panels at 15°+ allow rain to wash dust away naturally. Flat or near-flat installations (common on flat roofs without proper tilt frames) accumulate dust much faster and require manual cleaning.

Add an annual panel clean to your maintenance budget: R500–R1 500 depending on system size and roof access. Use clean water and a soft cloth or squeegee — never abrasive materials or pressure washers.